What’s the Key Issue?
The Global Positioning System provides accurate timing information to many of our critical systems – power grid, communications, financial markets, emergency services, and industrial control to name a few. It also includes the ability to transmit the proper date and time to a receiver by supplying the receiver with the current week and the current number of seconds into the week. This allows the receiver to translate the date and time into a more typical format – day, month, year, and time of day.
However, the field that contains the week number is a 10-bit binary number. This limits the range of the week number to 0 – 1023, or 1024 total weeks.
GPS week zero started January 6, 1980. The 1024 weeks counter ran out and rolled over on August 21, 1999. The week counter then reset to zero, and it has been recounting ever since. The next time the counter will reach week 1023 and rollover to zero is on April 6, 2019.
GPS Time as defined in the legacy GPS navigation message (ICD-200), uses 10 bits to count GPS Week Numbers. This representation can only cover a finite period of 1024 weeks (19.7 year epoch).
- GPS Time started on Jan 6, 1980
- The first GPS Time Epoch ended on Aug 21/22 1999.
- GPS Time is presently in its second Epoch which will end on April 6, 2019
- It’s up to the user receiver to resolve this week number ambiguity
- Newer receivers fully compliant with GPS ICD should handle this event OK
- In the Future the Modernized GPS Navigation (CNAV and MNAV) message has a 13-bit week number, which for all practical purposes solves this ambiguity